1. Field of the Invention
The present invention relates to an optical waveguide system.
2. Related Art
Surface plasmon is an electromagnetic wave mode observed in the boundary surface between a metal and a dielectric material or air, and has characteristics that electromagnetic waves are localized in the regions in a size of the light diffraction limit or smaller along the surface and are reinforced. Taking advantage of such characteristics, applied researches have been recently made in the fields of nanophotonics, biotechnology, and the like. In the leading edge of those researches, metal nanoparticles of nano-order size having a higher area ratio with respect to volume are applied as plasmon excitation media. Such electromagnetic wave mode is known as localized surface plasmon. Particularly, a method for transmitting plasmon polariton, using the metal nanoparticles, as electromagnetic wave signals through an optical waveguide is being actively studied. To realize such an application, various methods for generating plasmon polariton are being studied, as well as the research of methods for manufacturing metal nanoparticles are advanced.
Also, near-field optical probes having various shapes and characteristics are being studied in order to reduce the loss when converting the light from a light source into plasmon polariton (refer to Reference 1 (T. Saiki, S. Mononobe, M. Ohtsu, N. Saito, and J. Kusano, Appl. Phys. Lett. 68, 2612 (1996)), for example). Reference 1 discloses a double tapered near-field optical probe that has high resolution and high optical propagation efficiency. In this probe, the sharp tip angle in the region where the cross-sectional diameter of the sharp tip portion becomes equal to or smaller than the optical wavelength is made greater than the sharp tip angle in the vicinity of the probe root so as to make the distance to the tip end as short as possible.
The most common method for generating plasmon polariton in the field is a method for exciting plasmon polariton by converting propagated light of a light source into near-field light. In this method utilizing a tapered near-field optical probe, a nanometer-order location control device and a processing technique are necessary. There is also a problem with a near-field light emitting device such as a near-field optical probe in that its conversion efficiency is as low as an order of 10−4.
Surface plasmon antennas, each of which has a metal thin-film surface fine-processed into a concentric circle, are also being actively studied, as opposed to tapered near-field optical probes. With such a surface plasmon antenna, surface plasmons are concentrated to a localized region (a region having a diameter of 300 nm, for example) in a center of concentric circles, so as to generate near-field light. The results of measurement carried out in the region where the surface plasmons are concentrated with use of a Si nano-photodiode show efficiency improvement by one order of magnitude (refer to Reference 2 (T. Ishi, J. Fujikata, K. Makita, T. Baba, and K. Ohashi, “Si Nano-Photodiode with a Surface Plasmon Antenna” Jpn. J. Appl. Phys. 44, L364-L366 (2005)), for example).
Likewise, there has been a technique for gathering surface plasmons generated from propagated light through a diffraction grating to a localized region (one end of a nanodot coupler in this case) with use of a surface plasmon polariton capacitor formed with several metal fine particles arranged in semicircle (refer to Reference 3 (W. Nomura, M. Ohtsu, and T. Yatsui, Appl. Phys. Lett. 86, 181108 (2005)), for example). There is also a known technique for exciting surface plasmons in a film structure according to an attenuated total reflection (ATR) method for generating surface plasmons in a metal thin film applied to or located in the vicinity of a prism total-reflection face through the near-field light generated from propagated light on the prism total-reflection face (refer to Reference 4 (H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, (Springer-Verlag Berlin Heidelberg) 1988), for example).
As a more direct coupling technique, there is proposed a technique for exciting plasmons in one-dimensionally arranged nanoparticles by irradiating propagation light gathered by a lens to one end of the nanoparticles (refer to JP-A 2006-171479 (KOKAI), for example).
There is also proposed a highly refractive embedded and tapered “high-mesa” optical waveguide (refer to JP-A 2002-311262 (KOKAI), for example).
By such top-down fine processing techniques, however, it is necessary to prepare an expensive fine processing apparatus such as an electron beam lithography apparatus, and the production costs become higher when optical waveguide systems are mass-produced. Furthermore, the wavelength controllability of the light introduced into the waveguide is limited by the structure of each optical waveguide system. Also, phase matching is necessary to couple propagated light to near-field light.
As opposed to the above-described top-down techniques, there is provided a technique by which quantum dots are mixed as a gain material with metal nanoparticles or a inorganic hybrids (refer to Reference 5 (D. Parekh, L. Thylen, and C. J. Chang-Hasnain, “Metal nanoparticle and quantum dot metamaterials for near resonant surface plasmon waveguides”, Nano-Optoelectronics Workshop, 2007, I-NOW '07. International, Jul. 29, 2007—Aug. 11, 2007, p.p. 150-151), for example). With the technique disclosed in Reference 5, it is possible to locate quantum dots three-dimensionally mixed in a structure having metal nanoparticles dispersed therein, and thus to reduce the light loss in the waveguide by the quantum dots (the gain material). Accordingly, the waveguide distance is made longer according to Reference 5. Further, Reference 5 discloses that the emission wavelength selectivity defined by the size and material of the quantum dots contribute to the gain.
As described above, a system that converts light propagated from light sources of various wavelengths into near-field light, and couple the near-field light to an optical waveguide becomes complicated in structure. Also, phase matching is necessary, and the conversion efficiency is low. Furthermore, the light emitted from the light source has no directivity along a desired propagation direction from an end of the waveguide.